Many products, in particular food products, are sensitive to the presence of oxygen and the loss or absorption of water. Packaged products with this sensitivity are susceptible to deterioration because of exposure to oxygen or absorption of moisture. Packaging materials which limit oxygen exposure to food articles, for example, help to maintain the quality of the food articles and to reduce spoilage. The use of such barrier packaging thus keeps the article in inventory longer and thereby reduces restocking costs and waste. Attempts to solve this problem have led to the widespread use of oxygen barriers and/or moisture barriers in packaging materials. Many polymeric materials are known to act as barriers to oxygen or moisture. For example, typical moisture barriers include polyethylene and polypropylene. Representative oxygen barriers include poly(ethylene vinyl alcohol) (“EVOH”), poly(vinyl alcohol) (“PVOH”), polyamides (nylons), and blends of these materials. Poly(vinylidene chloride), vinyl chloride copolymers, and vinylidene chloride-methyl acrylate copolymers also are useful as moisture and oxygen barriers.
These conventional barrier materials, however, are expensive and have unstable structural characteristics or other deficiencies that make fabrication of packaging materials solely out of barrier materials difficult or undesirable. For example, EVOH, while having superior oxygen barrier properties, is not effective as a moisture barrier. Other barrier materials are prohibitively expensive to be used solely as a packaging material. To avoid these problems, it has become a common practice to use multilayer structures in which the amount of expensive barrier material may be reduced to a thin layer and used in conjunction with an inexpensive polymer on one or both sides of the barrier layer as structural layers. The use of multilayer structures also helps to protect the barrier layer from deterioration by structural layers. Multilayer products, however, can be expensive to produce. Further, multilayer articles can present difficulties in recycling because the different polymer components are difficult to separate. In addition, blending the recovered scrap polymer or “regrind” with virgin polymer often will cause unsatisfactory haze or opaqueness because of the incompatibility of the virgin materials with the regrind.
The shortcomings of conventional barrier polymers also may be overcome by using a blend of the barrier polymer with another polymer. Unfortunately, as noted above, many blends of barrier polymers and other thermoplastic polymers are immiscible and are opaque or hazy. Such blends are not satisfactory for applications requiring clarity such as, for example, beverage containers.
Polyester polymers such as, for example, poly(ethylene terephthalate) (“PET”), are commonly used in packaging applications. PET has a number of properties that make it useful as a packaging material, including acceptable carbon dioxide barrier properties for soft drinks packaged in bottles containing multiple servings. However, improvements in the carbon dioxide barrier of PET are needed for soft drinks packaged in smaller bottles and in its oxygen barrier, which is not well-suited for packaging oxygen sensitive products such as, for example, beer, citrus products, tomato-based products, and aseptically packed meat. Poly(ethylene naphthalate) (“PEN”) is 3-10 times more effective as a barrier than PET but is more expensive.
Multilayer structures can be used to improve the gas barrier characteristics of PET. For example, polymers that have excellent oxygen barrier (also referred to as “passive barrier”) or scavenging properties (also referred to as “active barrier”) may be combined with PET to produce a layered structure consisting of the individual polymers. These multilayer structures, however, are expensive to produce. Blends of barrier polymers with PET also have been used to improve the oxygen barrier of packages but, as noted above, often have poor transparency and are not suitable for many packaging applications. The poor transparency of blends also makes it difficult to recycle manufacturing scrap from polymer blends into virgin polymer.
Copolyester films and extrusion blow molded (“EBM”) bottles are often desired for toughness, and are commonly used instead of PET for extrusion blow molding and film applications. These applications often require barrier that is comparable to that of oriented PET. Unfortunately, however, the barrier properties of copolyesters are inferior to oriented PET. Multilayer structures can be produced by coextruding a thin, barrier film into the center of a thicker bulk structure to improve the overall barrier. To be economical, however, EBM and film processes typically require that high levels (up to 80%) of regrind (i.e., flash and trim) are reprocessed. Unfortunately, typical barrier materials are not miscible with copolyesters and blends of these barrier polymers with polyesters often show a high level of haze and poor clarity. The haze level of the overall film structure, therefore, is increased to unacceptable levels when scrap polymer (i.e., regrind) is incorporated back into the primary layer.
A polymer blend that provides good passive and/or active barrier properties, is economical, and can be recycled efficiently is needed in the art. Such blends should be transparent, contain thermoplastic and barrier polymers that provide a high barrier for oxygen, water, and carbon dioxide, and can be used economically in article forming processes which incorporate a high level of regrind. In addition, there is need for barrier and thermoplastic polymer compositions that can be used to economically produce multilayered articles having high transparency and can tolerate a high level of regrind.